Prior Inoculation with Type B Strains of Francisella tularensis Provides Partial Protection against Virulent Type A Strains in Cottontail Rabbits.

Vienna R Brown,Danielle R Adney,Richard A Bowen,Francisco Olea-Popelka,Dennis W Metzger

doi:10.1371/journal.pone.0140723

Vienna R Brown, Danielle R Adney + Show 3 more

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https://doi.org/10.1371/journal.pone.0140723

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Journal: PloS one	Publication Date: Oct 16, 2015
Citations: 1	License type: CC BY 4.0

Affiliation: Colorado State University

Abstract

Francisella tularensis is a highly virulent bacterium that is capable of causing severe disease (tularemia) in a wide range of species. This organism is characterized into two distinct subspecies: tularensis (type A) and holarctica (type B) which vary in several crucial ways, with some type A strains having been found to be considerably more virulent in humans and laboratory animals. Cottontail rabbits have been widely implicated as a reservoir species for this subspecies; however, experimental inoculation in our laboratory revealed type A organisms to be highly virulent, resulting in 100% mortality following challenge with 50–100 organisms. Inoculation of cottontail rabbits with the same number of organisms from type B strains of bacteria was found to be rarely lethal and to result in a robust humoral immune response. The objective of this study was to characterize the protection afforded by a prior challenge with type B strains against a later inoculation with a type A strain in North American cottontail rabbits (Sylvilagus spp). Previous infection with a type B strain of organism was found to lengthen survival time and in some cases prevent death following inoculation with a type A2 strain of F. tularensis. In contrast, inoculation of a type A1b strain was uniformly lethal in cottontail rabbits irrespective of a prior type B inoculation. These findings provide important insight about the role cottontail rabbits may play in environmental maintenance and transmission of this organism.

Highlights

IntroductionFrancisella tularensis (F. tularensis) is an intracellular, zoonotic bacterium, infection which induces tularemia [1]
Francisella tularensis (F. tularensis) is an intracellular, zoonotic bacterium, infection which induces tularemia [1]. This organism is capable of causing severe disease in a wide variety of species and, due to its low infectious dose and high virulence (the LD50 for some type A strains of F. tularensis has been found to be as low as one colony-forming unit in mice), is classified as a Tier 1 Select Agent by the Centers for Disease Control and Prevention (CDC) [2, 3]
F. tularensis is classified into two subspecies: tularensis and holarctica which are referred to as type A and B respectively, and are responsible for the vast majority of human tularemia cases [4,5]

Summary

Introduction

Francisella tularensis (F. tularensis) is an intracellular, zoonotic bacterium, infection which induces tularemia [1]. Type B has been found in North America and is the only species endemic in Europe; this organism is associated with mosquito-borne transmission and an aquatic cycle, primarily involving beavers, muskrats, and voles [12]. These reservoir species become infected and contaminate waterways via their carcasses and urine which serve as a route of infection for mosquito larvae, other aquatic mammals, and humans [13, 14]. Type B strains of F. tularensis cause mortality in 7% of human cases [10, 11]

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